Faraday rotation is a Magneto-optical phenomenon. This is an interaction between light and a magnetic field that is applied to a medium or sample through which the light passes. The Faraday effect causes a rotation of the plane of polarization of the light source. A conventional Faraday rotation spectroscopic (FRS) system includes a polarized light source and an absorption cell. An AC magnetic modulation field is applied to the sample parallel to the laser-beam direction. If a paramagnetic sample (such as NO molecule, for instance) is present inside the sample chamber, the applied magnetic field rotates the polarization of light source due to the Faraday effect, and the amount of polarization rotation is related to spectroscopic sample parameters, concentration of target species and optical path-length.
Faraday rotation spectroscopy (FRS) has drawn much attention recently because it can provide ultra-sensitive and selective detection of paramagnetic species in gas phase (e.g. NO, NO2, O2). There are two main approaches to perform generation and detection of Faraday rotation signals: (i) AC modulated magnetic field is used to modulate the magneto-optical properties of the sample and a non-modulated laser is used to optically detect the FRS signal (AC-FRS), and (ii) A static (DC) magnetic field is used to interact with the sample gas and a wavelength-modulated laser is used to optically detect the FRS signal (DC-FRS). The major drawback of DC-FRS is that its sensitivity is usually limited by parasitic Fabry-Perot interference fringes generated by multiple optical paths between the laser source and photo detector.
To suppress these effects a balanced photodetection can be employed for DC-FRS measurement. This however requires two well matched photodetectors that are costly and difficult to obtain in the mid-infrared spectral region. In contrast, the signal in AC-FRS can be effectively distinguished from those parasitic effects even if single detector element is used. This is possible, because the modulated magnetic field allows for selective modulation of the magneto-optic properties of the sample and the unwanted magnetically inactive background is suppressed. Therefore in AC-FRS the total system noise consists primarily of photo detection system noise and laser noise, thus it can provide higher sensitivity than DC-FRS. In both AC- and DC-FRS the applied modulation process results in modulated polarization of the transmitted laser beam, which is then detected using a polarizer (analyzer) followed by a photodetector. In order to reject a broadband noise, a phase-sensitive lock-in amplifier is used to demodulate the signal specifically at the modulation frequency or its harmonics.
One important problem that occurs in AC-FRS is related to electro-magnetic interference (EMI) that leads to substantial signal offsets measured by the lock-in amplifier. The EMI occurs due to relatively high currents that are needed to produce required magnetic field by the electromagnetic solenoids. The EMI pick-up is difficult to control and typically occurs in the detector circuitry or laser driving electronics. This significantly deteriorates the system sensitivity and most importantly impacts the system long-term stability, because the amplitude of the pick-up tends to drift over time. Another important problem in AC-FRS is a difficulty of achieving high modulation frequencies with electromagnetic solenoids. driven by high currents Thus the AC-FRS measurements are typically performed at frequencies of several kHz. This low frequency range is dominated by large relative intensity noise (RIN) of laser sources (RIN shows 1/f dependence), and the sensitivity of current AC-FRS systems is strongly limited by RIN. Improved FRS systems and methods are desirable.